A high voltage transmission system comprises, depending on whether it is single-phase, double-phase or three-phase, one or more high voltage cables. Typically, the high voltage cable comprises a conductor and a screen surrounding set conductor. Furthermore, the high voltage cable is provided with an insulating and water-proof outer coating. In order to avoid the entry of water or water vapours through the outer coating to the cable conductor the outer coating preferably consists of metal or a metal sheet in combination with an outer layer of PE or PVC (layer coating). The avoidance of water intrusion is of particular significance, since intruded water accelerates the aging processes of the insulating materials of the cable made of cross-linked polyethylene and thus leads to an early damage or failure of the cable.
Water monitoring systems and water monitoring methods are therefore used in order to detect the water intrusion into the cable and to determine the location of the water intrusion. Typically, one or more water sensing wires are provided in the conductor screen and a measuring circuit performs voltage drop measurements at the water sensing wire or water sensing wires because a water intrusion into the cable will cause a change of the isolation resistance of the water sensing wire and thus different voltage drops at the water sensing wire. In order to perform the voltage drop measurements typically the measuring circuit injects a measuring current into the water sensing wire.
Whilst the provision of the water sensing wire in the cable has the advantage of allowing the detection and locating of a water intrusion into the cable, of course the water sensing wire itself is an additional electrical component in the cable. This additional electrical component as well as the manner in which the water intrusion measurement is carried out (i.e. how the current, voltage or resistance measurements are carried out), can alter the electrical characteristics of the cable in certain conditions. That is, a water intrusion into the cable, which is provided with a water sensing wire should not produce, merely due to the provision of the water sensing wire, additional side effects in comparison to a cable which is not provided with such a water sensing wire.
More specifically, since the water sensing wire extends in the screen either parallely to the conductor or is wound around the conductor voltages may be induced in the water sensing wire due to the currents flowing in the conductor. In case of a water intrusion such induced voltages cause currents flowing through the water intrusion location leading to a local increase of temperature and can cause electro/chemical effects.